Ab initio study of MgH2 formation

Even if there is considerable literature dealing with structure and properties of MgH2 compound there are still some uncertain details about nature of bonding governing its formation and decomposition. In order to better understand the processes essential for absorption and desorption of MgH2, ab initio DFT based calculations of rutile MgH2 compound, elemental hcp-Mg, and three different hypothetical hcp-Mg-derived hydrides are performed. Our findings show that all structures are unstable, and that MgH (Wurtzite) is a closest possible candidate for intermediate phase between the hcp-Mg and MgH2 at 1:1 stoichiometry. An alternative hydration pathway is suggested, including promotion of hcp-Mg to bcc-Mg and consecutive transformation to rutile MgH2 by means of hydrogen incorporation into Mg matrix. Rutile MgH2 calculations with various hydrogen vacancies concentration are performed. Calculation shows that at high hydrogen concentration close to 1:2, stable substoichiometric hydride is possible. Calculation also shows that high vacancy (low hydrogen) concentration favors bcc-Mg2H over rutile Mg2H structure.